The present invention relates to a device or method for measurement of the concentration of a substance. More particularly, it relates to a device or method for measurement of the concentration of a substance which is diffusable through inert membranes.
Devices of the above mentioned general type are known in the art. One of such devices is disclosed, for example, in the U.S. Pat. No. 4,263,115. The device disclosed in this patent has a reduction electrode for the oxygen composed of a noble metal and connected to a voltage source, and a reference electrode annularly disposed about the reduction electrode. A ligand membrane is provided in front of the reduction electrode and contains a cationselective carrier and is permeable to hydrogen ions. A closure membrane to seal the electrodes against the outside space is provided permeable to oxygen and impermeable to water. An aqueous electrolyte containing the ligand cation of the ligand membrane is disposed between the ligand membrane and the closure membrane. The known arrangement is advantageous for the concentration measurement especially because with the provision of a protective membrane on the electrode, larger electrodes can be used, which is thereby cheaper, simpler to manufacture and handle, and have smaller impedance, without the problem of diffusion gradient which conventionally takes place in large electrodes and so that damaging substances which do not belong to measurement reaction can be retained far from the electrode. It is desirable to form these arrangements so as to enable it to measure also the concentration of other important, especially physiologically important, substances.